I was wondering the same thing myself. When I was planning my London-Berlin trip laste easter I was surprised to find out LH do not provide a direct link - capital city to capital city, very strange. Only BA and Buzz have flights. I did chose LH anyway, I had to change at Dusseldorf on the way, and on my return in Frankfurt.
It seems ridiculous for LH not to have such a service. A twice daily 737/320 service atleast, or even British Midland could operate the service and codeshare with LH(like they do on most UK-germany services). Seeing as there are direct services from most other main German cities(Hamburg, Dusseldorf, Munich, Stuttgaert, Frankfurt, Hannover etc) I think Berlin definetly needs an LH link to heathrow.